1. Field of the Invention
The present invention relates to a fuser unit with a cleaning mechanism and an image forming apparatus having the same.
2. Description of the Related Art
In recent years, the printing process speed of an image forming apparatus is increasing. As the printing process speed increases, improvement in the capability of removing residual toner (cleaning capability) of a cleaning mechanism of a fuser roller in a fuser unit is required. The fuser unit performs fusing with a heat roller and a pressure roller rotating while in contact with each other with pressure. The cleaning mechanism is a mechanism for cleaning the heat roller and/or the pressure roller. Hereinbelow, the heat roller and/or the pressure roller will be described as fuser rollers. When the printing process speed increases, an amount per unit time of toner residing on the fuser rollers increases. The toner residing on a surface of the fuser rollers or the toner once removed remains in a cleaning unit. A problem such that the toner hardened in the non-conductive state or a power saving mode damages the fuser rollers in current-carrying operation after that becomes obvious. That is why improvement in the cleaning capability is required.
Therefore, in place of the conventional cleaning mechanism (a fuser cleaning mechanism using felt or roller), a “web cleaning system” is often used (refer to, for example, Japanese Unexamined Patent Application Publication No. 2003-107952). The web cleaning system is a mechanism using a web of sheet in a band shape as a cleaning member and winding the web sheet intermittently so that the sheet in a fresh state comes into contact with the fuser roller. The cleaning mechanism of the web cleaning system can clean the fuser rollers until the end of the web sheet without largely deteriorating the initial cleaning capability.
The direction of winding the web sheet after cleaning has to be opposite to the movement of the peripheral face of the fuser roller for the following reasons. The residual toner on the fuser roller is removed at the tip of a nip part in which the web sheet comes into contact with the surface of the fuser roller, that is, on the upstream side in the direction of movement of the peripheral face of the fuser roller. The removed toner is taken by the web sheet on the upstream side in the nip part. The web sheet is wound in the direction in which the removed toner does not pass through the nip portion, that is, the direction opposite to the direction in which the peripheral face of the fuser roller moves. If the web sheet is wound in the forward direction, the toner taken by the web sheet passes through the nip portion and returns to the fuser roller side during the passage, and it is feared that the surface of the roller becomes dirty.
In a low temperature state, there is the possibility that hardened toner damages the surface of the roller. Further, when the web sheet partially taking toner and whose thickness becomes nonuniform passes through the nip portion, the press contact force to the fuser roller of the web sheet becomes nonuniform. As a result, the frictional force received from the fuser roller also becomes nonuniform, and it causes a wrinkle in the web sheet. When a wrinkle occurs in the web sheet, the contact to the surface of the fuser roller becomes nonuniform, and it causes poor cleaning. When a state where a wrinkle occurs continuously, the web sheet may be broken.
FIG. 8 is a diagram schematically showing the configuration of a fuser cleaning mechanism 201 of a conventional web cleaning system. FIG. 8 shows an example of the case where the fuser cleaning mechanism 201 is provided for a heat roller 202.
The fuser cleaning mechanism 201 includes a web sheet 203 as a band-shaped cleaning member for cleaning a surface, a press roller 204 provided for pressing the web sheet 203 on a surface of the heat roller 202, a feed roller 205 used in a state where the web sheet 203 is wound around its peripheral face and sequentially feeding the wound web sheet 203 from its tip, and a wind roller 206 for winding the web sheet 203 fed from the feed roller 205 and cleaned the surface of the heat roller 201.
When the heat roller 202 rotates in the rotative direction shown by an arrow 207, toner 208a adhered in a fused state on the peripheral face of the heat roller 202 in a slide contact part with the web sheet 203 in a stationary state is removed. The removed toner 208a is stored on the upstream side of the contact part between the heat roller 202 and the press roller 206 in an almost fused state. Toner 208b is the toner stored as described above and is stored in a gap formed by the web sheet 203 and the surface of the heat roller 202.
When the toner 208b accumulated in the gap reaches to a certain amount, the wind roller 205 rotates in the direction of an arrow 209 and winds the web sheet 203 only by predetermined length. The accumulated toner 208b is therefore detached from the surface of the heat roller 202 in a state where the toner 208b is adhered to the web sheet 203.
The web sheet which comes into contact with the fuser roller receives a force in the direction along the travel of the peripheral face by the friction with the peripheral face of the fuser roller. The force is in the direction opposite to the direction of winding the web sheet. As the printing process speed increases, the peripheral speed of the fuser roller also increases. As a result, the frictional force received from the fuser roller by the web sheet is also enhanced. Hitherto, by the tension from the wind roller for winding the web sheet, backward travel of the web sheet is prevented. However, as the printing process speed increases, in some cases, the tension becomes insufficient. Due to elongation of the web sheet, the toner accumulated on the upstream side of the nip portion enters the nip portion. When the toner enters the nip portion, it causes dirt or damage in the fuser roller and a wrinkle in the web sheet. To prevent such problems, for example, it is necessary to excessively wind the web sheet in consideration of the “elongation”.
On the other hand, the web sheet as a cleaning member has to be resistant to the heat of the fuser unit and pressure-contact with the roller surface to be cleaned. Preferably, the web sheet is porous to absorb and hold residual toner. Usually, the web sheet is wound only by length according to the use of a predetermined period and loaded in a fuser unit in a state where the web sheet can be fed. The web sheet has to be thin so that it can be housed in the fuser unit. A member suitable for such a use is not common but special and expensive. It is therefore preferable that the web sheet be finely wound in order to avoid wasteful use. In other words, it is preferable to wind the web sheet little by little while maintaining the cleaning capability so as to save the web sheet. From this viewpoint, it is unpreferable to wind the web sheet excessively.
Consequently, a mechanism for preventing backward travel of the web sheet without wasting the web sheet is in demand.
In the following cases, it is preferable to prevent the travel of the web sheet irrespective of the peripheral speed of the fuser roller. When paper jam occurs in a state where a sheet to be conveyed is sandwiched between the heat roller and a pressure roller in a fixing unit, the ejection direction varies according to the state where the sheet resides. In the case of removing the sheet in a state where a most part resides on the upstream side (transfer side) of the fuser roller, usually, the operator pulls the sheet to the transfer side. On the other hand, in the case of removing the sheet in a state where a most part resides on the downstream side (paper ejecting part side) of the fuser roller, the operator pulls the sheet to the paper ejection side. When the sheet sandwiched in the nip portion is pulled out, the fuser roller rotates along the direction of pulling the sheet. As the fuser roller rotates, a force also acts on the web sheet which is pressure-contact with the fuser roller. When the web sheet moves in the case where the sheet is pulled to the paper ejection side, toner absorbed by the web sheet at the tip of the nip portion enters the nip portion. When the web sheet moves in the case where the sheet is pulled to the transfer side, the web sheet is fed uselessly.
A mechanism for preventing travel of the web sheet at the time of removing the paper jam in the fuser unit is in demand.